1. Field of the Invention
This invention relates to heat engines operating on the Ericsson cycle which comprises the steps of isothermal compression, regenerative heat addition, isothermal expansion, and regenerative heat removal. More particularly, it relates to an improved Ericsson open cycle air engine where regenerative heat addition is effected solely by burning fuel in the expanded low pressure exhaust stream.
2. Description of Related Art
The Ericsson cycle, disclosed in Ericsson U.S. Pat. No. 13,348 (1855), U.S. Pat. No. 14,690 (1856), and U.S. Pat. No. 431,729 (1890) consists of isothermal compression of the working fluid at a low temperature followed by: heat addition at constant pressure to a high temperature, isothermal expansion at the high temperature, and heat removal at constant pressure to the low temperature. The Ericsson cycle can ideally achieve the optimum thermodynamic efficiency of the reversible Carnot cycle, dependent only on the absolute values of the high and low cycle temperatures.
Practical Ericsson engines are of the open cycle type with either internal or external combustion. Ericsson's original engines had an external combustor producing hot gases which supplied heat to the working fluid via a heat exchanger or by directly heating the exterior of the high temperature expander cylinder. These engines had limited success because the materials of the time could not withstand the high temperatures needed to compete with the fuel economy of contemporary steam engines. Also, the complexity of the Ericsson valve mechanism was a disadvantage compared to that of the simpler contemporary Stirling cycle engines. Another, more significant, drawback to the external combustion Ericsson engine is that fuel and air enter the external combustor at ambient environmental temperature. The energy required to heat the combustion gases to the high cycle temperature is not available to the working fluid and is lost to the cycle. The potential of the external combustion Ericsson cycle to approach Carnot cycle efficiency is therefore compromised by the combustion efficiency.
In the internal combustion Ericsson cycle engine the combustion efficiency loss of the external combustion Ericsson cycle engine can be avoided by using the working fluid as the combustor air. Combustion is initiated in the high pressure/high temperature air stream between the regenerator and the, expander. In this way the air is preheated by the regenerator and the heating loss is minimized.
Previous internal combustion Ericsson cycle engines have been of the gas turbine or reciprocating type. The gas turbine version, used for large-scale power generation, is based on a Brayton cycle having a compressor with multiple intercooled stages, and an expander with multiple stages having intermediate reheaters. As the, number of intercoolers and reheaters is increased, the compression and expansion become more isothermal and the cycle approaches the Ericsson cycle.Lay, Joachim E.: "Thermodynamics", Charles E. Merrill Books (1963) p.572!. The turbine Ericsson cycle is impractical for all but large powerplants because of the high cost and complexity of the multiple stage turbines.
Reciprocating Ericsson cycle engines are more economical for small scale power generation. Fuel is injected into and burned with air, the normal working fluid, to achieve heat addition. Here, as in other open-cycle internal combustion engines, valves are required to admit and exhaust air and hot combustion gas streams. Top cycle temperatures and pressures are limited by the thermo-structural properties of valve materials.
An example of the internal combustion approach is the "Modified Ericsson.degree. Cycle Engine" disclosed in U.S. Pat. No. 4,133,172 (1979) to Cataldo. Here fuel is injected and burned in the high temperature/high pressure air stream between the Ericsson cycle regenerator and expander. Although combustion efficiency is boosted by preheating the combustion air by an exhaust gas regenerator, the combustion process becomes complex and is not everywhere continuous. Combustion occurs continuously in a primary combustor located between the regenerator and the expander. However, during the expansion, the expander inlet valve closes and isolates the primary combustor from the expander. Additional fuel must then be added via a second, intermittent, combustor to the expander to keep the expansion isothermal. This two-stage process requires two separate high-pressure fuel injection systems, one, continuous and the second synchronized with the downstroke of each individual expander. A proper amount of fuel must be injected into each stage to assure smooth running, maximize efficiency and minimize emissions. The required amounts vary continuously with load conditions, engine speed, and temperatures. The result is a very complex engine with a high potential for unsatisfactory exhaust emissions, particularly during transients. A further limitation of the Cataldo engine is that the expander inlet valves are exposed to the full flame temperature of the primary combustor. The inability of valves to tolerate such high temperatures--at high pressure--has historically limited the life of Ericsson engines.
It is the primary aim of this invention to overcome the disadvantages of current Ericsson cycle engines discussed above and to achieve long engine life, reduced emissions and ease of control by implementing the several objects listed below.